Cell identifiers typically have 2 contradicting targets. On the one hand, they have to be short in order to ease the detection for the terminal. It is noted that in most cases the cell identifiers are coded into the reference (pilot) signals which are used for cell detection. So they have to be read without knowing anything from a cell, without being able to estimate the channel, and without having synchronization to a cell. This makes the PCI detection a rather complex hypothesis test. Thus, the smaller the set of PCIs (=codes), the easier and faster the detection means.
On the other hand, they must be unique in a local neighborhood. A large set of PCIs (i.e. long identifiers) are needed to provide uniqueness for a sufficiently large area.
In the following, the latter requirement (unique in a local neighborhood, will be discussed in more detail. It implies 2 conditions:
First, it is obvious that adjacent neighboring cells must have different PCIs, otherwise they cannot be distinguished. Violation of this condition is called PCI collision.
FIG. 1 illustrates an example of the problem. “ECGI” (enhanced cell global identifier) is the LTE (Long Term Evolution) terminology for a global (i.e. unique) cell identifier, which is unique but unfortunately also too long for cell detection by simple decorrelation means on UE side. The terminal moving from ECGI1 towards ECGI3 both with identical PCI codes will interpret the reference signals (RS) of ECGI3 as an echo or multi-path component of the RS of the current serving ECGI1, i.e. it would not even realize that there is another cell (details are below). At some point it will drown in the interference of ECGI3 and therefore will suffer a failure (radio link failure).
Secondly, neighbors of neighbors must also have different PCIs, which is however less obvious. Violation of this condition is called PCI confusion.
FIG. 2 illustrates an example of the resulting problem, where the two cells ECGI1 and ECGI3 have the same PCI1. The UE goes from cell ECGI2 to ECGI3 and reports a measurement event that PCI1 is stronger than serving PCI2. The serving cell ECGI2 owns a neighbor relation with ECGI1 having the same PCI1 and initiates the handover preparation towards ECGI1 which will obviously result in a failure.
However, in reality it is not obvious at all which cells have a neighbor relation. In networks with a very dense deployment a single cell may have more than 50 neighbors. This is a result of unexpected propagation such as coverage overshots of far distant cells. So the expression “neighbor relation” has a more abstract meaning, far beyond a pure geographical meaning.
Finally, it is important to mention that a single cell (e.g. ECGI2) can only have cells on the neighbor relation table (NRT) which are uniquely identifiable, i.e. all PCIs within the NRT are different from ECGI2 and mutually different from each other. The problem is that also the ANR (Automatic Neighbor Relation) function is not able to detect neighbors having the same PCI as another neighbor. As seen in the upper simple example illustrated in FIG. 2, if there is a neighbor relation between ECGI1 and ECGI2, and the terminal would report PCI1, the serving cell ECGI2 must interpret it as ECGI1 which is listed in NRT.
Similar in the case of collision: Since PCI is expressed by the reference signals, the terminal must interpret the identical reference signal from ECGI3 as an echo of the RS of ECGI1. If the cells are synchronized and the signals are received within the cyclic prefix, the terminal would measure the sum of both signal strengths. If the ECGI3 reference signal is received outside the cyclic prefix, the terminal would experience it as interference. In any case, the UE will not be able to detect that there is another cell and thus would not even send a measurement report.
Thus, PCI collision and confusion are very hidden problems. They lead to connection failures (radio link failures/handover failures), and the root cause is very difficult (or impossible) to detect.
For the ANR (automatic neighbor relation) feature, the 3GPP specification has defined a feature where the serving cell can instruct a terminal to read the ECGI of an already reported neighbor. This allows the base station to set up a missing neighbor relation, in case an unknown PCI is reported. However, in the case of the two above discussed problems, where either no PCI is reported at all (collision) or a well-known PCI is reported (confusion), i.e. ANR is not suitable. Irrespective of limited capability, the ANR feature is rather complex for the terminal, and it takes quite some time (since the terminal needs to synchronize to the new cell, read the system information and then return back to the serving cell to report it). It is unlikely that the terminal has stable channel to both cells for such a long time.
Another approach to tackle PCI conflict issues is exploiting failure statistics from OAM perspective (Operation and Maintenance). If there are for a considerable long period unusually many failures where no obvious root cause is found, one could derive that there might be a PCI problem. However, this will have some limited confidence, and it will take quite some time to detect it.